That depends on how deep you want to go, Falcon 4 is a hard core sim, I mean it has a very steep learning curve, if you want to learn all the systems. LOMAC is a sim-game, like was said earlier, not nearly as deep. Graphics in Lomac are much better, but if you get hi-tiles for Falcon, it is a very good looking sim. I have tried them both and I got rid of LOMAC, I still fly Falcon and have enjoyed it very much, I still do.
Another option that was mentioned is The Strike Fighters series, there's so many add-ons for it, and they are for the most part free, and the level of detail is amazing. If you want the more modern aircraft then by far the one to choose is Wings over Europe, theres a Wings over Vietnam, and Strike Fighters:Project 1, and if you are into WW1 then First Eagles is the newest offering from TK. For me the choice was easy...the mighty F-4, in all her glory!
I Don't know how much time you want to invest in learning, so I would suggest that you look at all the different options, also how much of a system you have is another thing to consider, LOMAC is very hardware intensive, Falcon isn't to bad, and the Thirdwire sims are much easier than Lomac, and the graphics can be tweaked to suit you on all of them.